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By Jesse Yoder, PhD, Flow Research, Inc.
While variable area (VA) flowmeters
are limited in their functionality, they cost far less than most other types of
flowmeters. When users are looking for a simple, low-cost solution, they
continue to consider VA meters. While
they can measure flowrate, they are sometimes used when a flow/no-flow
determination is desired. They are
very effective at measuring low flowrates, and can also serve as flow/no-flow
indicators. VA meters do not require
electric power, and can safely be used in flammable environments.
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The history of variable area
meters goes back to 1908 when they were invented by German engineer Karl
. At that time they were called “rotameters,”
named after the rotating float that was originally a component of these meters.
Felix Meyer recognized the commercial potential of Kueppers’ invention,
and in 1909 founded “Deutsche Rotawerke” in
. The product invented by Karl
Kueppers was the first variable area flowmeter with a rotating float.
The German company Deutsche
Rotawerke was the forerunner of the company that later became known as the Rota
Company. Originally, Meyer called
his products “rotamesser.” In
1995, Yokogawa purchased the Rota Company and named the resulting company Rota
Yokogawa. Rota Yokogawa still
manufactures its variable area meters, which it also calls rotameters, in
In the meantime, the GEC
Crawley Company in
, began manufacturing the first glass variable area meters, and registered the
name Rotameter as a trademark in the
. This name still exists as a
trademark in the
, but has passed down through a number of companies, including KDG Instruments
and Solartron Mobrey. In March 2009,
Emerson Process Management acquired Solartron Mobrey, presumably for its level,
density, and flow computer products. As
part of the acquisition, Emerson Process acquired the trademark to the name
“rotameter” in the
The terms “rotameter” and
“variable area meter” have become synonyms over the years.
It appears, though, that Yokogawa has the rights to this name in
and that Emerson Process has the rights to the name in the
. On the other hand, the name
‘rotameter’ has become a generic term for variable area flowmeters, so its
status as a true trademark is open to question.
This is what happened to ‘xerox,’ which is now used generically to
describe any photocopy.
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Most variable area (VA)
flowmeters consist of an upright, tapered measuring tube that contains a float.
Fluid flows through the tube from bottom to top.
The upward force of the fluid raises the float in the tube.
The float is counterbalanced by the force of gravity.
The point at which the float stays constant indicates the volumetric
flowrate, which can be often read on a scale on the meter tube. Variable area
meters vary according to their float shape, and the material the float is made
Variable area meters come in
plastic, glass, and metal. Their
cost varies with their material of construction.
Metal-tube meters are generally highest in cost, and are designed for
high pressure and high temperature applications.
Plastic meters are typically lowest in cost, followed by glass flowmeters.
Some metal tube meters range can exceed $1,000 in price. A group of VA meters
called purgemeters have been designed for low flow applications.
For most plastic and glass VA
meters, the flowrate can be read from the scale on the tapered meter tube.
For metal VA meters, the position of the flow is magnetically transmitted
to an analog indicator. These meters
also have a scale indicating flowrate, and the position of the analog indicator
displays the flowrate. These VA
meters still rely on a float to indicate flowrate, and they also need to be read
manually, like most of the tube design meters.
Metal VA meters can be used for liquid, gas, and steam applications.
VA flowmeters are used in
research and laboratory environments and in the process industries to measure
the flow of gases and air at low flowrates.
They are also used when a visual indication is sufficient, and to check
on the performance of other meters. VA meters, including plastic meters, are
also used for OEM applications.
Plastic meters are applicable
for water, gas, and air applications. The
scales are separately marked for water and air applications, and they are
available in both English and metric units.
They are most widely used when cost is the main consideration, and high
accuracy is not required.
KROHNE began making variable
area meters in
in 1921. Today KROHNE makes a wide
range of plastic, glass, and metal VA meters that can be used for a variety of
applications. Some typical
applications for KROHNE’s meters include hygienic and chemical processes,
water and wastewater, and offshore applications. KROHNE quotes accuracies
ranging from 1 percent of measured value to 4 – 6 percent of measured value,
depending on the model.
Today Yokogawa makes a range of
metal, glass, and plastic VA meters it calls the RA Series.
meter has a transmitter that supports both HART and Profibus PA.
Having a transmitter output is still the exception for most VA meters,
but is becoming more common as companies want to combine the advantages of low
cost with automation. In addition to
indicating flow at a point in time, the RAMC has a totalizing option.
Brooks Instrument is another
major supplier of variable area meters. The
company offers both low-flow and high-flow glass tube meters.
The company quotes 2 percent, 3 percent, and 10 percent accuracy for its
high-flow glass tube meters, depending on the model.
It quotes accuracies of 5 percent and 10 percent on the low-flow glass
tube meters. Brooks has a line of
armored metal tube VA meters called the MT Series.
These meters have corrosion resistant options for metering aggressive
fluids. Transmitters with 4-20mA and
HART are offered as options for remote monitoring.
Blue-White Industries offers
primarily acrylic and polysulfone VA meters.
Polysulfone is a type of plastic that is tolerant of high temperatures.
The company offers both tube and block models.
The block type meters are rectangular instead of round, and are designed
for panel mounting. Blue-White
quotes flowrates of 0.25 to 100 gallons per minute (GPM) on some of its machined
acrylic meters. The largest line
size of these meters is two inches.
Variable area flowmeters are
considered to be traditional technology meters.
They are the least expensive and least sophisticated of the traditional
meters. They are relatively simple
in construction when compared to a turbine or positive displacement meter.
Yet variable area meters have maintained their market size over the
years, even though this size is less than that of turbine or positive
One reason why VA meters have
maintained their presence in the market is that suppliers have offered
improvements such as transmitter outputs, and have built VA meters designed for
specific applications. This often
means choosing particular materials of construction, along with a different
shaped tube or float. This increases
the versatility of VA meters and makes them more appealing to end-users with
Another reason for their growth
is that they fill a genuine need in the market.
In some cases end-users simply need a flow/no-flow indication, or they
are satisfied with 2 percent or 5 percent accuracy.
Variable area meters may also do well in regions where cost is a major
consideration and high accuracy is not always required.
Depending on application, VA meters may do well in
. KROHNE is positioned to supply
these regions with its variable area manufacturing facility in
. Another major VA supplier
positioned to sell into the Asian region is Tokyo Keiso of Japan.
Variable area meters have a
fascinating past, and they could have a bright future.
How bright their future will be depends on how much research and
development suppliers decide to put into them.
By incorporating more advanced features such as transmitter outputs, more
varied materials of construction, different float designs, and more industry
approvals, VA suppliers can potentially position themselves for expansion.
At the moment, much research and development money is going into Coriolis
and ultrasonic meters. That is not
likely to change, but if VA suppliers can continue to improve their products,
and develop higher accuracies, the variable area flowmeter market has a chance